Projection lens systems (also referred to herein as "projection systems") are used to form an image of an object on a viewing screen. The basic structure of such a system is shown in FIG. 3, wherein 10 is a light source (e.g., a tungsten-halogen lamp), 12 is illumination optics which forms an image of the light source (hereinafter referred to as the "output" of the illumination system), 14 is the object which is to be projected (e.g., a LCD matrix of on and off pixels), and 13 is a projection lens, composed of multiple lens elements, which forms an enlarged image of object 14 on viewing screen 16. The system can also include a field lens, e.g., a Fresnel lens, in the vicinity of the pixelized panel to appropriately locate the exit pupil of the illumination system.
Projection lens systems in which the object is a pixelized panel are used in a variety of applications, including data display systems. Such projection lens systems preferably employ a single projection lens which forms an image of, for example, a single panel having red, green, and blue pixels.
Pixelized panels, specifically, LCD panels, come in various sizes depending upon the type of projection system in which they are to be used. Large LCD panels, e.g., panels having a diagonal of from about 5.0 inches (about 130 millimeters) to about 10.6 inches (about 270 millimeters), can be effectively employed in producing high resolution color images since such panels can have a high pixel count while still maintaining a pixel size which is large enough for reliable manufacture. In this regard, it should be noted that for a full color image from a single LCD panel, the number of pixels needed is three times that required for a monochrome image, thus making for small pixel sizes unless large LCD panels are used.
There exists a need in the art for projection lenses for use with pixelized panels which simultaneously have at least the following properties:
(1) a long focal length; PA1 (2) a relatively simple construction, i.e., a relatively small number of lens elements; PA1 (3) a high level of color correction; PA1 (4) low distortion; and PA1 (5) low sensitivity to temperature changes. PA1 (6) the ability to operate (focus) over a wide range of magnifications (conjugates) while maintaining an efficient coupling to the output of the illumination system and a high level of aberration correction (hereinafter referred to as the "lens' focus range"); and PA1 (7) the ability to provide a range of magnifications for any set of conjugates in the lens' focus range while again maintaining an efficient coupling to the output of the illumination system and a high level of aberration correction (hereinafter referred to as the "lens' zoom range"). PA1 (A) a first lens unit (U1) having a positive power P.sub.1, said first lens unit comprising in order from its image side to its object side: PA1 (B) a second lens unit (U2) having a negative power P.sub.2, said second lens unit being spaced from said first lens unit by an axial distance D.sub.12 and comprising in order from its image side to its object side: PA1 (1) the first lens subunit comprises (most preferably, consists of) a positive lens element (L.sub.1P) and a negative lens element (L.sub.1N), the positive lens element being on the image side of the first lens subunit and the negative lens element being on the object side of that subunit, said positive and negative lens elements being separated by an axial distance D.sub.1P1N ; PA1 (2) L.sub.1P is composed of a low dispersion plastic material, e.g., acrylic, and has at least one aspherical surface; PA1 (3) L.sub.1N is composed of a high dispersion glass material, i.e., a flint glass; PA1 (4) L.sub.1N has spherical surfaces; PA1 (5) the second lens subunit consists of a single lens element (L.sub.2); PA1 (6) L.sub.2 is composed of glass and has spherical surfaces; PA1 (7) the second lens unit consists of L.sub.2P and L.sub.2N ; PA1 (8) L.sub.2P is composed of glass and has spherical surfaces; PA1 (9) L.sub.2N is composed of a low dispersion plastic material, e.g., acrylic, and has at least one aspherical surface; PA1 (10) the projection lens consists of only five lens elements; and/or PA1 (11) the projection lens satisfies some or, preferably, all of the following numerical limitations: PA1 (a) an illumination system comprising a light source and illumination optics which forms an image of the light source, said image being the output of the illumination system; PA1 (b) a pixelized panel which comprises the object; and PA1 (c) a projection lens of the type described above, said projection lens having an entrance pupil whose location substantially corresponds to the location of the output of the illumination system.
For certain applications, it is desirable for a projection lens having the foregoing properties to also having the following two additional properties:
For a large pixelized panel, the use of a long focal length allows the field of view of the projection lens to be maintained in a range which facilitates aberration correction, e.g., the semi-field of view of the lens can be less than 15.degree.. Also, a long focal length permits a long throw distance which is desirable in large auditoriums.
A relatively simple construction is desirable from a cost, weight, and size point of view. Large numbers of lens elements consume more raw materials, weigh more, and are more expensive to build and mount. Accordingly, a lens with a minimum number of lens elements is desired.
As is well known in the art, lenses which are capable of zooming generally employ numerous elements in a complex structure to simultaneously provide focal length changes and aberration correction. Accordingly, properties (2) and (7) above work against one another in arriving at a suitable lens design.
In this regard, it has been found that a mini-zoom range, e.g., a zoom range in terms of image height at the viewing screen (object height in the tables) of 15% or less (.+-.7.5% or less from the center point of the zoom range), is sufficient to meet the great majority of the zooming needs of users of LCD projection lens systems. Moreover, it has been further found that such mini-zoom ranges can be achieved using simple lens designs which employ at most one additional lens element and, in many cases, no additional lens elements than a fixed focal length design which can achieve a similar image quality over a similar focus range. In this way, the incompatibility of properties (2) and (7) can be overcome.
More particularly, by limiting the zoom range, it has been found possible to design the lenses of the invention by: 1) designing a non-zooming lens which will achieve the desired focus range, 2) selecting a lens unit of such a design which will be moved during zooming, and 3) reoptimizing the design with the selected lens unit being moved to achieve the desired zoom range. In practice, it has been found that the zooming version of the lens requires the addition of at most one lens element and, in many cases, no lens element, to achieve limited zooming while maintaining a desired focus range.
Property (3), i.e., a high level of color correction, is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. These problems are typically most severe at the edges of the field.
All of the chromatic aberrations of the system need to be addressed, with lateral color, chromatic variation of coma, and chromatic aberration of astigmatism typically being most challenging. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection systems employing cathode ray tubes (CRTs) a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction is thus needed from the projection lens.
The use of a pixelized panel to display data leads to stringent requirements regarding the correction of distortion. This is so because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, an undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center. Moreover, projection lenses are often used with offset panels, the lenses of the examples being designed for such use. In such a case, the distortion at the viewing screen does not vary symmetrically about a horizontal line through the center of the screen but can increase monotonically from, for example, the bottom to the top of the screen. This effect makes even a small amount of distortion readily visible to the viewer.
Low distortion and a high level of color correction are particularly important when an enlarged image of a WINDOWS type computer interface is projected onto a viewing screen. Such interfaces with their parallel lines, bordered command and dialog boxes, and complex coloration, are in essence test patterns for distortion and color. Users readily perceive and object to even minor levels of distortion or color aberration in the images of such interfaces.
In order to produce an image of sufficient brightness, a substantial amount of light must pass through the projection lens. As a result, a significant temperature difference normally exists between room temperature and the lens' operating temperature. In addition, the lens needs to be able to operate under a variety of environmental conditions. For example, projection lens systems are often mounted to the ceiling of a room, which may comprise the roof of a building where the ambient temperature can be substantially above 40.degree. C. To address these effects, a projection lens whose optical properties are relatively insensitivity to temperature changes is needed.
One way to address the temperature sensitivity problem is to use lens elements composed of glass. Compared to plastic, the radii of curvature and the index of refraction of a glass element generally change less than those of a plastic element. However, glass elements are generally more expensive than plastic elements, especially if aspherical surfaces are needed for aberration control. They are also heavier. As described below, plastic elements can be used and temperature insensitivity still achieved provided the powers and locations of the plastic elements are properly chosen.
Optional property (6), i.e., the ability to efficiently operate over a wide range of magnifications (a large focus range), is desirable since it allows the projection system to be used with screens of different sizes and halls of different dimensions without the need to change any of the components of the system. Only the object and image conjugates need to be changed which can be readily accomplished by moving the lens relative to the pixelized panel. The challenge, of course, is to provide efficient coupling to the output of the illumination system and a high level of aberration correction throughout the operative range of magnifications.
The ability to change the magnification of the image for a given set of image and object conjugates, i.e., the ability to zoom, has similar benefits. In this case, the changes in magnification are used for fine tuning of the image to, for example, fully fill a viewing screen.
It should be noted that while properties (6) and (7) above, i.e., focusing and zooming, both involve magnification changes, the magnification changes are achieved in fundamentally different ways. Thus, during focusing, the focal length of the lens remains substantially constant as the image and object conjugates change, and the magnification change is a result of the change in the ratio of those conjugates. During zooming, on the other hand, the focal length changes while the image and object conjugates remain constant, and the magnification change is a result of the change in the focal length. (Note that in Table 1 below, the focal lengths shown include the Fresnel lens which remains fixed during focusing (and zooming). As a result, the focal length of the lens appears to change during focusing, when in fact it is remaining constant for the lens elements which move during focusing. Also, the lenses of the tables include means for accommodating for changes in the focus of the lens resulting from zooming. In particular, the tables show some changes in conjugates during zooming which achieve these focus corrections.) The projection lenses described below achieve all of the above requirements and can be successfully used in producing relatively low cost projection lens systems capable of forming a high quality color image of a pixelized panel on a viewing screen.